Under the 3GPP standards, a NodeB (or an eNB in LTE) is the base station via which mobile devices connect to the core network. Recently the 3GPP standards body has adopted an official architecture and started work on a new standard for home base stations (HNB). Where the home base station is operating in accordance with the (Long Term Evolution) LTE standards, the HNB is sometimes referred to as a HeNB. A similar architecture will also be applied in the WiMAX network. In this case, the home base station is commonly referred to as a femto cell. For simplicity, the present application will use the term HNB to refer to any such home base station and will use the term NodeB generically to refer to other base stations (such as the base station for the macro cell in which a HNB operates). The HNB will provide radio coverage (for example, 3G/4G/WiMAX) within the home, small and medium enterprise, shopping Malls etc and will connect to the core network via a suitable public network (for example via an ADSL link to the Internet) or operator network and in the case of the 3GPP standards, via an optional HNB gateway (HNB-GW) which typically will aggregate traffic from several HNBs.
An HNB may be configured to operate using one of a plurality of access modes, namely: ‘closed’ in which the HNB operates as a closed subscriber group (CSG) cell; ‘hybrid’ in which the HNB operates as a CSG cell where at the same time, non-members are allowed access; and ‘open’ in which the HNB operates as a normal (non-CSG) cell.
When the HNB is in the hybrid access mode, and services cannot be provided to a CSG member due to a shortage of HNB resources, established communications of non-members via a CSG cell may be diverted to another cell. Furthermore, to minimise the impact of non-member established communication on CSG members, the network may allow the data rate of established communication of non-members to be reduced.
At the time a connection is established between a mobile device (also known as User Equipment, UE) and a HNB, the HNB sends an initial UE message to the mobility management entity, MME in the core network. Where the HNB is configured to operate in a closed or hybrid mode, the initial UE message includes a closed subscriber group identifier, CSG-Id, for the UE. The MME performs access control based on the CSG-Id received in the initial UE message and stores specific information for the UE in a database, such as an MM or EPS bearer context. The MME actions for UE specific procedures, such as account charging, CSG membership change or paging optimisation are different depending on whether the UE is connected to a closed or hybrid cell. Details of these UE specific procedures are described in the 3GPP Standard TS 23.401 version 10.1.0 (2010 Sep. 29): “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access” and in 3GPP Standard TS 23.060: “General Packet Radio Service (GPRS); Service description; Stage 2”.
A recent proposal has been made that if a UE moves from one HNB to another HNB belonging to the same closed subscriber group, then the MME does not need to perform access control for the new HNB. However, the inventors have identified a problem with this proposal when one HNB is operating in the closed subscriber group access mode and the other HNB is operating in the hybrid mode. Thus there exists a need to adapt the recent proposals to overcome this problem.
Although for efficiency of understanding for those of skill in the art, the invention will be described in detail in the context of a 3G system (UMTS, LTE), the principles of the invention can be applied to other systems (such as WiMAX) in which mobile devices or User Equipment (UE) communicate with one of several base stations with the corresponding elements of the system changed as required.